What Is the Difference Between AC and DC? . Both AC and DC describe types of current flow in a circuit. In direct current (DC), the electric charge (current) only flows in one direction. Electric charge in alternating current (AC), on the other hand, changes direction periodically. The voltage in AC circuits also periodically reverses because the current changes direction.
Most of the digital electronics that you build will use DC. However, it is important to understand some AC concepts. Most homes are wired for AC, so if you plan to connect your Tardis music box project to an outlet, you will need to convert AC to DC. AC also has some useful properties, such as being able to convert voltage levels with a single component (a transformer), which is why AC was chosen as the primary means to transmit electricity over long distances.
Read More : Power Formulas in DC and AC 1-Phase and 3-Phase Circuits
Electric Current
Electric current is the motion or flow of free electrons in a conducting material under the influence of potential difference. The material that contains free electrons is called conductors and they are used for conducting an electric current.
The free electrons existing in a material are excited when a voltage or potential difference is applied and they flow in a specific direction i.e. from high potential to the low potential. The high potential or voltage is denoted by positive (+) sign and the low potential is denoted by negative (–) sign and they form the polarity of the electric current.
Based on the direction of motion of the electron or electric current, it is classified into two main types; Alternating Current (AC) and Direct Current (DC)
Read More : Role of Capacitor in AC and DC Circuit
What is an Alternating Current (AC)?
In alternating current, the electric charge flow changes its direction periodically. AC is the most commonly used and most-preferred electric power for household equipment, office, buildings, etc. It was first tested based on the principles of Michael Faraday in 1832 using a Dynamo Electric Generator.
Alternating current can be identified in a waveform called a sine wave. In other words, it can be referred to as a curved line. These curved lines represent electric cycles and are measured per second. The measurement is read as Hertz (Hz). AC is used in powerhouses and buildings because generating and transporting AC across long distances is relatively easy. AC is capable of powering electric motors which are used in refrigerators, washing machines, etc.
Read More : What Is the Difference Between AC and DC Motors?
Generation
When a coil or a loop of wire is placed in a varying magnetic field, it induces an electric current in the coil. This principle is applied in the devices called Alternators which is used for the generation of Alternating current.
The alternator is made up of a coil that rotates (by any means such as water or steam turbine) inside a stationary magnetic field. The rotation of the coil varies the magnetic field lines affecting the coil; therefore an electric current is induced in the coil. Since the rotating coil reverses the polarity of the magnetic field, the electric current and voltage induced in the coil changes its direction periodically.
Waveforms
The magnitude of the alternating current and voltage varies continuously through time. It oscillates between its maximum peak point and its minimum peak point along a common reference point. The resultant waveform could be a sine wave, square wave, triangular wave, saw tooth, etc. the most common waveform of AC that we use in our homes is sine wave.
Read More : Analog to Digital Converters (ADCs)
Frequency and Phase
We already know that alternating current has a specific frequency and we know that frequency affects the reactance of the capacitor and inductor. Therefore, the alternating current introduces reactance to the circuit. The reactance causes a phase difference between the voltage & current waves. We can also say that this is the reason the power factor is involved in only AC systems. Because the definition of power factor is cos (θ) where θ is the phase difference between the voltage waveform and current waveform
The phase difference is the difference with respect to the time shift between the two AC waves. In such cases, the magnitude of one wave lags behind the magnitude of the other wave. This causes power loss in the circuit. In order to deliver full power to the load, the AC voltage and current needs to be synchronized (or in-phase). So the power factor fluctuates between cos 0° (power factor = 1, phase difference of 0°) and cos 90° (power factor = 0, phase difference of 90°).
AC Current, Voltage, Resistance & Power Formulas
AC Current
Single Phase AC Circuits
- I = P / (V x Cosθ)
- I = (V/Z)
Three Phase AC Circuits
- I = P / √3 x V x Cosθ
Read More : What is the Difference Between AC and DC Resistance?
AC Voltage
Single Phase AC Circuits
- V = P/(I x Cosθ)
- V = I / Z
Three Phase AC Circuits
- VL= √3 VPH or VL = √3 EPH [Star Connection]
- VL= VPH [Delta Connection]
AC Resistance
- Z = √(R2+ XL2)… In case of Inductive Load
- Z = √(R2+ XC2)… In case of Capacitive Load
- Z = √(R2+ (XL– XC)2… In case of both inductive and capacitive Loads.
AC Power
Single Phase AC Circuits
- P = V x I x Cosθ (in Single phase AC Circuits)
Three Phase AC Circuits
Active Power
- P = √3 x VL x IL x Cosθ (in Three Phase AC Circuits)
- P = 3 x VPh x IPh x Cosθ
- P = √ (S2 – Q2)
- P =√ (VA2 – VAR2)
Read More : What is the Difference Between AC and DC Generator?
Reactive Power
- Q = V I Sinθ
- VAR = √ (VA2 – P2)
- kVAR = √ (kVA2 – kW2)
Apparent Power
- S = √ (P + Q2)
- kVA = √kW2 + kVAR2
Complex Power
- S = V I
- S = P + jQ … (Inductive Load)
- S = P – jQ … (Capacitive Load)
Where
- I = Current in Amperes (A)
- V = Voltage in Volts (V)
- P = Power in Watts (W)
- R = Resistance in Ohm (Ω)
- Cosθ = R/Z = Power Factor
- Z = impedance = Resistance of AC Circuits
- IPh = Phase Current
- IL = Line Current
- VPh = Phase Voltage
- VL = Line Voltage
- XL = Inductive Reactance = 2πfL…Where L = Inductance in Henry.
- XC = Capacitive Reactance = 1/2πfC… Where C = Capacitance in Farads.
Read More : Can We Use AC Circuit Breaker for DC Circuit and Vice Versa?
What is Direct Current (DC)?
Unlike alternating current, the flow of direct current does not change periodically. The current electricity flows in a single direction in a steady voltage. The major use of DC is to supply power to electrical devices and also to charge batteries. Example: mobile phone batteries, flashlights, flat-screen television and electric vehicles. DC has the combination of a plus and a minus sign, a dotted line or a straight line.
Everything that runs on a battery and uses an AC adapter while plugging into a wall or uses a USB cable for power relies on DC. Examples would be cellphones, electric vehicles, flashlights, flat-screen TVs (AC goes into the TV and is converted into DC).
Generation
Direct current can be generated in many ways. The same AC generation method can be used to generate DC by connecting a device called a commutator. The commutator is a rotating device that ensures to keep the current flow in a single direction.
Read More : Why Cant We Store AC in Batteries instead of DC?
The Direct Current is usually generated by using batteries and cells. The batteries include a chemical substance that releases electrons upon chemical reaction and supply it to the electrical circuit.
The Alternating current can also be converted into DC by using a device called a rectifier.
Waveform
The direct current does not have any specific waveform because it only flows in a single direction. If you connect a DC to an oscilloscope, it will show a straight line. However, if the voltage is pulsating, say in a digital circuit that purely runs on DC voltages, the signal waveform might appear as a pulse train or square waves. But the waveform never falls below 0V.
DC Current, Voltage, Resistance & Power Formulas
DC Current
- I = V/R
- I = P/V
- I = √P/R
DC Voltage
- V = I x R
- V = P / I
- V = √ (P x R)
DC Resistance
- R = V/I
- R = P/I2
- R = V2/P
Read More : Difference between AC Drives and DC Drives
DC Power
- P = IV
- P = I2R
- P = V2/R
Where
- I = Current in Amperes (A)
- V = Voltage in Volts (V)
- P = Power in Watts (W)
- R = Resistance in Ohm (Ω)
Storage and Conversion between AC and DC
We need both types of electric current in our daily life application. Digital devices such as smartphones, laptops and computers, etc. run on DC while our home and kitchen appliances such as fans, lights, and mixer, etc. run on AC.
The Alternating current and direct current are interchangeable. They can be easily converted from one form to the other. The device that converts AC to DC is called Rectifier while the device that converts DC to AC is called Inverter. We use both of them to convert between the power supplies according to our needs.
The outlets in our home provide AC supply but when we need to power a DC device using the same outlet, we use a rectifier (such as the power supply in PC or the power adapter in a laptop cable). It helps us using the same power source to power both types of devices. And we can also use the DC supply of batteries to power AC appliances using Inverters.
Read More : How to Wire ON/OFF Delay Timer for AC and DC Loads?
But there is a limitation of the alternating current i.e. the electric current can only be stored when it is in DC form. Therefore, the AC is converted into smooth DC before charging a battery such as in cellphones.
The charge storage provides mobility and wireless capability to the device. It is also used as an emergency backup in harsh conditions to power crucial equipment such as in hospitals etc.
Voltage Conversion and Transmission
The transmission lines experience power loss (I2R) in the form of heat due to the amount of current flowing through them. In order to decrease the amount of current, we increase the voltage to maintain the same power delivered (P = I*V).
In AC, the voltages can be easily converted between high and low voltages using a device called a Transformer. We use Step-Up Transformers at the generation stations to bump up the voltages for transmission over long distances. Also, the same voltages are brought down to safe levels for domestic or commercial uses using the Step-Down Transformer usually seen on utility poles.
There is very little loss in high voltage DC transmission and it requires only two wires but its maintenance and conversion between high and low voltage is very costly so it was never adopted. The DC voltage is dangerous than AC because the AC fluctuates while the DC is a constant stream of current and it will never let you go.
Read More : How to Design a Solar Photovoltaic Powered DC Water Pump?
Main Differences between AC and DC
The following table shows the comparison and key differences between alternating current “AC” and direct current “DC”.
| Characteristics | Alternating Current – AC | Direct Current – DC |
| Definition | The electric current that flows back and forth periodically. | The electric current that flows in only forward direction |
| Symbol | 
|
|
| Direction of current | It is bidirectional i.e. it can flow in both forward and reverse direction. | It is unidirectional and it flows in only one direction i.e. forward |
| Voltage and current | The current and voltage varies continuously. | The current and voltage is constant. |
| Polarity | There is no polarity in AC because it fluctuates. | There is fixed polarity in DC marked by Positive (+) and Negative (-) signs |
| Swapping Terminals or Polarity | Swapping the source terminal will not affect the circuit | Swapping the source terminal may damage the circuit. |
| Frequency | The frequency of the alternating current is usually 50 or 60 Hz | The frequency of the direct current is 0. |
| Complex Impedance | AC introduces reactance to the circuit so there is complex impedance. | The DC circuit has pure resistive loads. So the impedance is purely resistive |
| Power Factor | The power factor in AC is always equal to or between 1 and 0. | The frequency is 0 so the power factor is always 1. |
| Generation | The AC is generated using Alternator. | It is generated by using a commutator with a generator, using solar panels and using a chemical reaction in batteries and cells. |
| Waveform | The AC exists sine, square, triangular and saw tooth waveform, etc. | It exists in a single line or pulse wave. |
| Conversion | A rectifier is used for converting it into DC | An inverter is used for converting it into AC |
| Storage | It cannot be stored | It can be directly stored. |
| Transmission | There are some losses in transmission over long distances. | It has very low transmission losses at high voltage over a long distance. |
| Transmission lines | It requires a minimum of 3 separate conductors in its transmission | It only requires 2 conductors in its transmission |
| Transmission cost & maintenance | It is expensive but maintenance and conversion between voltages is easier than DC | It is cheaper but its maintenance is quite dangerous and costlier then AC |
| Hazard | The AC is less dangerous than DC because it reaches to 0V at some regular intervals. (not to be played with at high voltage) | The DC is very dangerous and life-threatening then AC because it maintains a constant current flow. |
Read More : Is Lightning AC or DC?
Difference Between AC and DC
- Ac current changes its direction during flow while the DC current does not change its direction during flow and remains constant.
- The AC current has a frequency that shows how many times the direction of current flow changes during flow while the frequency of the direct current is zero as it does not change the direction of flow.
- The power factor of AC is 0 to 1 While DC is Constant Zero.
- The AC current is generated by the alternator while the Photovoltaic cells, generators, and batteries generate DC current.
- The AC load can be capacitive, inductive, or resistive but the load on DC is always resistive.
- The DC current graph has a constant line showing magnitude and direction is constant while the AC current can be a sinusoidal wave, square wave, or triangular wave.
- The AC is converted into DC using a device named rectifier while the DC is converted into AC named inverter.
- AC is widely used in industrial equipment and consumer electronics like AC, Freezer, Cooler, washing machines, lights, and fans; While DC is used in electronic gadgets and small devices like clocks, laptops, cell phones, and Sensors.
- Ac can be transmitted over long distances with some loss while DC can be transmitted very long distances with very low loss using HVDC
Read More : What Is an AC-DC Converter?
Frequently Asked Questions – FAQs
Why can’t AC be stored in batteries instead of DC?
It is important to understand that batteries do not store the energy directly in them. They store electrical energy in the form of chemical energy. The positive terminal of an AC source is connected to the positive terminal of the battery and the negative terminal of an AC source is connected to the negative terminal of the battery. The current starts to flow. But, AC changes its polarity and there is no actual supply of the energy. This is because the positive half cycle cancel outs the negative half cycle. If this process continues, it can damage the battery. Therefore, AC is not stored in batteries.
What are the advantages of AC over DC?
Following are the advantages of alternating current over direct current:
- AC is less expensive and easy to generate than DC.
- AC can be transmitted across long distances without much energy loss, unlike DC.
- The power loss during transmission in AC is less when compared to DC.
Why is the use of AC voltage preferred over DC voltage?
There are two reasons why the use of AC voltage is preferred over DC voltage:
- The loss of energy during the transmission in AC voltage is less when compared with the DC voltage and this makes its installations easy when the transformers are at distance.
- AC voltage has the advantage of stepping up and stepping down as per the requirement.
What is wattless current?
Wattless current is defined as the current in an AC circuit when the average power consumption is zero.
What will be the value of the power factor in the circuit when it is at resonance?
The value of the power factor in the circuit when it is at resonance will be 1.
Give reasons for a loss in energy in the transformer.
Following are the reasons because of which there is a loss in energy in the transformer:
- Hysteresis
- Eddy current loss
- Leakage of flux
Related Topics
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Short-Circuits In Ac And Dc Systems Volume 1
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What Is an AC-DC Converter? Understanding Its Role in Modern Technology
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AC vs. DC Transmission Systems: Key Differences, Advantages, and Applications in Power Lines
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Why Are DC Motors Used in Trains and Not AC Motors? | Essential Insights for Railway Systems
